TechTalk: Wattmeter Use

Watt's Going On?
Troubleshooting with a
Wattmeter

A wattmeter is essential
for station maintenance.
Whether you're operating
a Travelers Information
Station (TIS), a Highway
Advisory Radio (HAR)
System or
an Emergency Advisory AM
Station, a wattmeter is
a valuable tool for
installing and
maintaining your
information radio
station. The wattmeter
is used to tune the
station's antenna and to
determine if the antenna
and transmitter are
working properly (by
allowing the measurement
of the system's forward
and reflected power).

There are a couple of
ways to use a wattmeter.
Both the SX100
Wattmeter and the
SX200 model now
provided on all new
information radio stations are
passive units, placed
between the transmitter
and the antenna. They
may be inserted for test
purposes, or they may be
hard-wired into the
system for convenience.

On the back of the
meter, find two UHF
coaxial termination
points. Route the coax
from the lightning surge
arrestor and antenna to
the connector, labeled
ANT. From the
connector labeled TX,
route the coaxial cable
to the transmitter.

Measurement steps
include . . .
With the wattmeter now
placed in series and the
transmitter off,
measure the forward RF
power. Here's how:

Set the meter's
FUNCTION switch to the
POWER position.

Set the RANGE switch
to the appropriate
setting. (For the SX100,
use the 30W range; and
for the SX200, use the
20W range).

Verify that the TX
and ANT output
connections are secure.

Turn on the
transmitter and turn up
the output power to 10
watts.

Set the POWER switch
on the wattmeter to FWD
(forward) and record the
reading. If you're using
the TR6000 transmitter,
the power-adjust dial is
labeled on the front of
the unit. If you have
the earlier TR20 (Phase II)
transmitter, open the
transmitter lid and
adjust the blue power
potentiometer, located
vertically just above
the power-switch. (A
properly tuned antenna
should allow a reading
of 10 watts.)

Record reflected
power by turning the
meter's POWER switch to
REF. This reading should
be less than 1, and less
than 1/10 the forward
power reading.

The higher the ratio of
forward to reflected
power.An
information radio system
with a high ratio
operates more
efficiently, resulting
in longer transmitter
life.

If your readings show
reflected power to be
nearly the same as
forward power, turn the
transmitter off. There
is a problem with the
antenna system. Confirm
that your coaxial,
feedline and groundplane
connections are secure.
If these connections are
secure, with a VOM
meter, verify that
coaxial lines have not
shorted or opened; and
check the antenna for
disconnects and damage.

If the wattmeter
indicates that there is
very low forward power
and reflected power,
there is internal
transmitter difficulty.
Turn the transmitter off
and place a dummy load
on the ANT connector
of the wattmeter (in
place of the coaxial
connection to the
lightning arrestor and
antenna.) Turn the
transmitter back on. If
the meter continues to
show low forward and
reflected power with the
dummy load in place,
service the transmitter.
(With a dummy load in
place and the
transmitter's power at
full, the reading should
be 10 watts forward
power and 0 watts
reflected.)